His industries minister instantly promised to "fast-track" British investment projects. But many Indian political parties, including the ruling Congress, parties of the left, and Muslim organisations, have sharply criticised the decision. Ever since the massacre of more than 1,000 people, mostly Muslims, in 2002, Modi has been accused by jurists, secular political leaders and civil society in India, as well as by the governments of many countries, of taking no action to prevent the violence, delaying the deployment of police to protect victims and failing to investigate and punish those responsible for the killings. Despite his repeated denials of each of these accusations, Modi is a potent symbol of militant defiance of secularism and constitutional-democratic principles, and remains greatly feared by Muslims.

The decision by the UK to resume contact with him is seen as a cruel blow to the causes of justice for the massacre's victims (which included three British citizens), and of non-discrimination against India's 180 million non-Hindus.

Up till now, British officials have followed a "working policy" of no contact with Modi's government "because of our concerns over what happened in Gujarat". Modi was also refused a visa by the US and EU. Modi's global isolation has helped to sustain domestic civil society pressure to bring the massacre's perpetrators to justice. This in turn encouraged the Indian supreme court to intervene, by asking the Gujarat government to reopen criminal cases closed for "lack of evidence", and transferring some trials to Maharashtra.

The new British stand has been rationalised on the ground that it would allow the UK "to discuss a wide range of issues of mutual interest and to explore opportunities for closer co-operation, in line with the … objective of improving bilateral relations …". In reality, it will deepen India's social and political rifts, and strengthen Modi's Hindu-chauvinist Bharatiya Janata party, for whose leadership he is making an aggressive bid.

Going by background briefings by British officials to the Indian press, the UK is attracted to "dynamic and thriving" opportunities in Gujarat, especially in "business", "science" and "education". Gujarat has emerged as a major investment destination thanks largely to the sweetheart deals Modi offers to businesses. Despite booming investments, Gujarat's social indices and poverty ratios remain appalling.

Evidently, the Cameron government doesn't want to lose out on Gujarat's business opportunities or support from Britain's prosperous Gujarati businessmen, described by the Foreign Office as "one of the most successful and dynamic communities in the UK" – even if that means sanctifying large-scale violence. The Foreign Office lamely added that it wants "to secure justice for the families of the British nationals who were killed … [and] support human rights and good governance …" That cannot be done by relaxing moral-political pressure on Modi.

The 2002 massacre was conducted by Hindu fanatics, who speared and burnt Muslims to death, besides raping hundreds of women. Some particularly ghastly incidents were documented by Human Rights Watch and an Indian magazine . A witness told Human Rights Watch that the belly of a pregnant woman was slit open by a mob with swords, and both she and her foetus were torched. At Naroda-Patiya, 97 people were massacred, including 35 children and 32 women, by a mob directed by former minister and Modi confidante Maya Kodnani, who has just been sentenced to 28 years in jail. Police claim they recorded these cases but could not pursue them because of lack of evidence. This is contested by eye-witnesses.

The massacre followed riots, which were themselves a reaction to a fire in a train, declared accidental by a railway inquiry, in which 59 Hindus died. According to well-corroborated accounts, including some 40 independent reports, including one by the Concerned Citizens Tribunal, composed of former senior judges, Modi had the bodies of the train-fire victims brought over a long distance to Ahmedabad, and displayed in a procession. Unsurprisingly, this provoked violence in Gujarat's charged climate. Modi has denied this, saying that the bodies were taken to a hospital to avoid any tensions, and that the violence was simply a natural "reaction" to the train fire.

According to independent accounts, denied officially, police stood by as the killing proceeded, and refused to register the crimes properly. The BJP, in power nationally, did not use the constitutional powers they have which could have helped restore confidence. (In India, if there is a breakdown of order in any state, the central government can sack the provincial government and impose its own rule until the situation improves and fresh elections can be held. In March 2002, a majority of Indian parties demanded that Modi be sacked and central rule imposed in Gujarat, but the BJP-led government refused.) Gujarat's climate has remained vitiated ever since, allowing Modi to win two state elections.

Despite official efforts to shield the culprits, and corrupt or destroy evidence, more than 110 people have been found guilty and sentenced by the courts – a small fraction of the culprits' number. Some evidence, as in Kodnani's case, was provided by conscientious policemen, including a detailed mobile-phone log that establishes frequent conversations between ministers, Hindu fanatics and police at specific sites.

Gujarat's victims have still not received justice. Thousands haven't been rehabilitated. Large numbers have been driven into ghettoes, and effectively disenfranchised within a communally polarised climate. Without justice, there can be no reconciliation or forgiveness.

Violence of this scale should be an international concern. It shouldn't be treated lightly simply because India has the trappings of a democracy with free elections. What makes the British decision politically odious is its timing: the Gujarat state election is just two months away.